This invention is directed to apparatus for generating a heated stream of oxygen and water vapor for use in connection with semiconductor processing equipment. In particular, the heated stream of oxygen and water vapor is used to grow thermal oxide films (S.sub.i O.sub.2) on semiconductor wafers and also is suitable for use in the deposition of oxide films.
The oxidation of silicon plays a significant part in the use of integrated silicon device technology. The role of the oxide began with the formation of diffusion masks, extended to the use of the oxide as a protective layer and moved on in significance to the use of oxide in integrated circuits. When silicon is exposed to an oxidizing gas, such as oxygen or water vapor, at an elevated temperature, an oxide forms on the surface. An oxide so formed is termed a thermal oxide and is grown on silicon surfaces prepared by chemical or mechanical surface preparation techniques.
While the formation of an oxide layer on silicon can be accomplished using high temperature water vapor or dry oxygen, wet oxygen oxidation wherein an oxygen stream is combined with water vapor provides additional flexibility in the layer formation. The water content of the gaseous stream supplied to the furnace containing the wafers is an important variable for determining oxide thickness. Uniformity and predictability of the characteristics of the oxide layer are dependent on maintaining the ratio of oxygen and water vapor constant.
Typically, the dry tank oxygen is passed through a water bath prior to being introduced into the oxidation furnace. The oxygen travels upwardly through the water vessel with the result that the ratio of oxygen to water is determined in part by the length of the path through the water. As the water level in the vessel changes, the path length of the oxygen gas changes as well and the ratio of the components of the output steam varies as well. The result is that the time required to form an oxide layer of desired thickness is often unpredictable.
An alternative to the passage of an oxygen stream through a water vessel over a period of several hours is the use of controlled hydrogen and oxygen streams combined in a pyrogenic reaction taking place within the furnace. This process typically requires expensive mass flow measurement and ratio control apparatus for each gas stream as well as an under-temperature shut down system. In addition, the safety standards for use of hydrogen in the clean room of a manufacturing facility require extensive shielding. The costs involved in conforming the facility to the local codes and ordinances reduces the desirability of this method of forming an oxide layer on silicon wafers.
Accordingly, the present invention is directed to apparatus for use in the generation of a wet oxygen stream for use in the manufacture of semiconductor components wherein the ratio of water and oxygen is constant during the generation of the stream. Further, the apparatus permits the establishment of the desired ratio prior to introduction of the oxygen and water into a vaporizing vessel. The ability to control the ratio over an extended period of many hours is found to improve the predictability and uniformity of oxide layer formed. The use of this apparatus eliminates the need to modify the facility to conform to safety requirements and thus provides a significant advantage when contrasted with apparatus using a stream of hydrogen to form an oxide layer.